Successful Rolling of 610 mm Seamless Steel Pipe on 720 Unit

Literature Overview

This technical report by An Ye and Zhou Yuan-Gui, published in Tianjin Metallurgy (2008, Issue 5, p. 76), announces the successful rolling of a 610 mm diameter by 26 mm wall thickness high-pressure boiler tube on the 720 seamless pipe rolling mill at the Tianjin Pipe Group's Third Round Pipe Company. This achievement followed the successful rolling of the 508 mm series of seamless pipes and represents a significant milestone in China's large-diameter seamless pipe manufacturing capabilities.

Core Technical Significance

The production of 610 mm diameter seamless steel pipes fills a critical gap in China's domestic seamless pipe manufacturing capacity. Large-diameter seamless pipes are essential for high-pressure boiler applications in power generation, petrochemical processing, and other heavy industrial sectors. Prior to this achievement, China relied on imports for such large-diameter seamless pipes, which posed both economic and strategic supply chain concerns.

Technical Parameters and Process Considerations

The successful rolling of this large-diameter, thick-walled seamless pipe required careful optimization of several process parameters. The rolling process for seamless pipes involves piercing the steel billet, then elongating and reducing the pipe blank through multiple rolling passes. For a 610 mm diameter pipe, the initial billet dimensions, piercing conditions, and rolling schedule must be carefully calculated to achieve the target dimensions without exceeding the material's deformation limits.

Material and Quality Requirements

High-pressure boiler tubes are subject to stringent material and quality requirements governed by standards such as GB 5310-2008 (Steel Tubes for High-Pressure Boilers), ASTM A199, and EN 10216-2. The material must exhibit adequate high-temperature strength, creep resistance, and resistance to oxidation and corrosion. Common grades include 20G, 12Cr1MoV, 15CrMo, and 12Cr2MoWV for different service temperature ranges.

The quality control requirements for large-diameter seamless boiler tubes include: (1) 100% eddy current testing or ultrasonic testing of the pipe body to detect internal and surface defects; (2) radiographic testing of the pipe ends; (3) hydrostatic testing at 1.5 times the maximum allowable working pressure; (4) mechanical property testing including tensile strength, yield strength, and impact toughness; (5) metallographic examination of the pipe cross-section to verify microstructure; and (6) chemical composition analysis to confirm grade compliance.

Process Analysis and Engineering Practice

The 720 rolling mill, named for its nominal rolling capacity, represents a significant piece of capital equipment in seamless pipe manufacturing. The successful expansion from 508 mm to 610 mm diameter production demonstrates the mill's capability to handle larger products through process optimization rather than equipment modification. This is a testament to the skill of the process engineers and operators who developed the rolling schedule for the larger diameter.

Key process considerations for large-diameter seamless pipe rolling include: (1) billet heating temperature and soaking time must be optimized to achieve uniform temperature throughout the large cross-section; (2) piercing force and speed must be controlled to avoid excessive deformation and surface defects; (3) the rolling schedule must account for the reduced strain rate and increased strain hardening at larger diameters; (4) cooling rate after rolling must be controlled to achieve the desired microstructure and mechanical properties; and (5) straightening and sizing operations must accommodate the larger diameter without introducing residual stresses or dimensional deviations.

Key Questions and Reflections

The successful rolling of 610 mm seamless pipes raises questions about further capacity expansion. Can the 720 mill be pushed to even larger diameters, such as 720 mm or beyond? The answer depends on the mill's mechanical design limits, including piercing pressure, rolling force capacity, and straightening capability. Process engineers should conduct systematic studies to determine the practical limits of the mill's production envelope.

Another important consideration is the consistency of product quality at the larger diameter. Large-diameter seamless pipes are more susceptible to壁厚不均匀 (wall thickness variation) and ovality, which can affect their performance in high-pressure applications. Engineers should implement statistical process control (SPC) to monitor dimensional accuracy and identify trends before they lead to quality deviations.

Study Insights and Implications

This achievement represents a significant advancement in China's seamless pipe manufacturing capabilities, reducing dependence on imported large-diameter boiler tubes. The successful process development from 508 mm to 610 mm demonstrates the potential for incremental capability expansion through process optimization. Engineers should document the process parameters and quality control procedures developed for the 610 mm product to facilitate future capacity expansion and to serve as a reference for similar mill upgrades at other facilities. The economic and strategic benefits of domestic production of large-diameter seamless pipes for high-pressure boiler applications are substantial, contributing to both industrial self-sufficiency and cost reduction in power generation and petrochemical sectors.